The present invention is directed generally to the coating arts and more particularly to an improvement in a coating apparatus for applying a coating of a shorter length than may normally be reliably achieved with such apparatus.
While the invention may find utility in other applications, the disclosure will be facilitated by reference to the application of a liquid glue medium to a moving substrate such as a paper carton, in a desired pattern. In the manufacture of folding paper cartons and the like it is desirable to apply lines or beads of glue to predetermined surfaces of the folded carton and in predetermined patterns for use in later erecting and assembling the carton. In some cases very short lengths of glue beads are required, either as part of a pattern or on relatively short surfaces.
Heretofore, primarily contact methods of gluing have been used in this application. In one contact method, a glue applicator in the form of a roller or like apparatus contacts the surface of each carton as a plurality of cartons move down a conveyor. However, this method of glue application has a number of drawbacks, for example it is messy and difficult to control.
In many instances, the limitations of the gluing wheel may be circumvented by replacing it with a controllable extruding applicator which rides upon the surface of the substrate and is maintained in contact with the substrate by a spring loading arrangement. Most often, this applicator is pneumatically operated with the pneumatic system being controlled in turn by an electrically actuated pilot valve. By driving the pilot valve from an appropriate electronic controller, it is possible to lay down a desired glue pattern. Such systems have been employed successfully in many applications, most notably the gluing of corrugated cartons. However, their usefulness at high surface speeds is limited by the relatively slow response of the electropneumatic components as well as by the dynamics of the suspension which maintains the contact between the applicator and the substrate. Moreover, this approach is not suitable for use with more delicate media, such as small folding cartons, which are normally run at higher speeds and which are unable to sustain the forces imposed by the spring-loaded applicator. In such situations, attempts to employ a surface-riding applicator are likely to result in damage to the cartons and/or frequent jamming of the cartons in the transport.
As a solution to the foregoing problems, non-contact gluing systems have been offered. In such systems, a nozzle-type applicator is generally spaced some distance above the surfaces of the cartons on the conveyor for applying glue in the desired pattern. However, problems have also arisen with the non-contact system. For example, some delay is inherent between the release of glue from the nozzle and its impingement upon the surface of the carton. Hence, proper timing of the opening and closing of a valve feeding the dispensing nozzle is required to compensate for this delay and ensure the proper disposition of the desired pattern upon the carton surface. As the speed of the conveyor increases, this compensation becomes increasingly important. One solution to the delay compensation problem is set forth in the copending application of James E. DeCamp, et al., Ser. No. 332,947, filed Dec. 21, 1981, now U.S. Pat. No. 4,408,562, for apparatus for applying a coating to a moving surface. Briefly, the non-contact coating system disclosed in that copending application utilizes a position sensor to ascertain the position of a moving substrate such as a carton to which a coating such as glue is to be applied in a desired pattern.
Preferably, the position sensor comprises a moving member, such as a wheel which is in contact with, and hence rotated by, a conveyor carrying the workpiece or carton. This wheel is coupled to a suitable electrical circuit which develops a known number of output pulse signals for a given distance travelled by the conveyor and hence workpiece or carton. The dispensing of a coating such as glue in a predetermined pattern, including necessary delay compensation, is controlled in accordance with the output pulses developed by this circuit responsive to the position sensor. Hence, both the control of the desired pattern and the required delay compensation are independent of the speed of the conveyor and workpieces or cartons carried thereupon.
It has been found that with the foregoing non-contact coating system, that there is a finite limit to the minimum length of a line or pattern of a coating or glue bead which may be reliably and consistently applied. Moreover, this minimum increases with increasing speed of the conveyor. This occurs primarily due to the characteristics of the glue control valve which must be fully opened and then fully closed to reliably deliver a consistent glue bead. Hence, the minimum length glue bead which may be deposited is dependent upon the minimum time in which a given valve will reliably cycle through a close-open-close sequence and this time varies somewhat with the characteristics of individual valves. Accordingly, the minimum length glue bead which may be consistently deposited is dependent upon the foregoing factors as well as upon the speed of the conveyor in such a system. An additional problem is presented by standard error or deviation inherent in such a system, that is, an error of plus or minus one distance unit which may occur in a counting arrangement coupled with the distance or position sensor. In practice, it has been found that the minimum length coating or glue bead which may be deposited is on the order of between substantially 1 inch and 1.4 inches at 1,000 feet per minute conveyor speed and is directly proportionate to conveyor speed. Attempts to set in a shorter length result in less than the required minimum time for valve operation described above and unreliable operation.
While the speed independent system of non-contact coating has proven accurate and reliable in most applications, in some cases it is still desirable to deposit a coating or glue bead of lesser length than the minimum length possible with the system as described above.